WESTFIELD, Iowa — While some treat snakes with revulsion, the Nature Conservancy has nothing but respect for the prairie rattlesnakes that slither through the tall grasses just south of town.

"While they are venomous, they do play a role in the overall system," said Susanne Hickey, Loess Hills project director for the Nature Conservancy, a private, nonprofit preservation group.

As many as 150 adult snakes roam a Nature Conservancy reserve in the 3,050-acre Broken Kettle Grasslands, Iowa's largest contiguous slice of native prairie.

It's the easternmost range of a snake that's found extensively west of the Missouri River and it's believed to be the only place they live in Iowa, a state not usually associated with harboring rattlers.

"Most people just about call you a liar when you tell them about the snakes," said Ed Schoenfelder, who farms just north of the Broken Kettle reserve. "But they're here."

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The Conservancy bought the land in the early 1990s to protect the grasses. While checking the property, staffers found the snakes, Hickey said.

"They feed on rodents and there are things that eat them," she said. "They're a critical part of the prairie system and we're trying to protect the whole complement of what's there."

Recently, the Conservancy added 15 acres to protect the snake's habitat from development and provide a buffer between the reserve and adjacent farms. The organization doesn't want housing or farming encroaching on the snake's habitat -- or the snake encroaching in areas where humans live.

Neither is particularly good for the other.

"We're trying to mitigate the negative interaction between snakes and people," said Scott Moats, who manages the Broken Kettle area.

"Some people wanted to build a house up here," Moats added while standing on a ridge that offered a view across the Big Sioux River into South Dakota. "It would have been built right on the hibernarium. We definitely didn't want that."

Still, that interaction happens. Schoenfelder's farmhouse sits at the base of the Broken Kettle hills and he runs cattle on land he rents from the Conservancy.

Working among the snakes has become a way of life for the 58-year-old farmer, who has lived on the property since he was an infant.

"We've had them on our front step," Schoenfelder said. "A lot of times they'll surprise you. But when fall comes and it turns cold, you know they've pretty well disappeared. Then about the first of May, you've got to start watching out again."

A prairie rattler once took up residence in a stack of hay bales on Schoenfelder's farm, apparently finding plenty of mice on which to feed. When it came time to breed, the snake moved on.

"They are pretty docile animals if you leave them alone," said Schoenfelder, who has never been bitten by one of the snakes. "They're here, I'm here, I guess I more or less put up with them. When night comes it's their time to travel, so I just go to the house and let them have the outside."

Prairie rattlesnakes, also known as the western rattler, are 35 to 45 inches long and require large blocks of prairie because they can travel 5 to 6 miles from their hibernation dens. Their diet includes rodents, birds and ground-nesting bird eggs.

They inject their prey with venom through fangs, then swallow it.

While the prairie rattler's bite is painful, the snake is too small to be fatal to humans who seek treatment, scientists say. Livestock often are bitten by prairie rattlesnakes, but most survive, although Schoenfelder said he once lost a calf to a snake bite.

Many times, in what's called a dry bite, a snake won't even inject venom into a human, said Dan Fogell, the herpetologist for the Nature Conservancy.

"They have complete control over when they inject venom," Fogell said. "If they're not striking a food item, they're usually not expending venom. It takes a lot of energy to produce venom and they won't waste it on something that's not food.

"A snake that would bite would have to be poked and prodded a lot."

Fogell, who teaches at Southeast Community College in Lincoln, Neb., estimates there are 120 to 150 adult prairie rattlers at Broken Kettle. He has tagged about 60 with a microchip that enables him to track the snakes. Fogell reads the microchip by passing an electronic scanner over the snake after catching it with a stick that has a clamp on the end.

The snakes live at the northern end of the Loess Hills, a line of silty bluffs that wind along the Missouri River valley on Iowa's western border. At Broken Kettle, the tallgrass prairie of the east mixes with the drier western prairie, perfect habitat for rattlers.

"If you walk around enough, you'll encounter several in a few days," Fogell said.

The only time Fogell saw a prairie rattler anywhere else in Iowa was three years ago in Palo Alto County in northern Iowa. He figures it was carried there in a load of hay or maybe in some pipe.

"It was just a fluke animal," he said.

Two other types of rattlesnakes live in Iowa. Timber rattlers are found in the northeast among the Mississippi River bluffs and there's a small population near Winterset in central Iowa, Fogell said. The massasuaga or swamp rattler lives in the Sweet Marsh Wildlife Management Area in Bremer County, also in northeast Iowa.